Communications system and method employing forward satellite links using multiple simultaneous data rates

ABSTRACT

A communication system and method for transmitting data to mobile platforms using multiple simultaneous channels operating at multiple data rates. Each mobile receiver has the capability to receive multiple transmit channels at multiple data rates. Aircraft report their position to a ground based controller which determines which communication channel can be operated without substantial loss of data. Data packets destined for each aircraft are preferably routed to the highest data rate channel that can be received by that aircraft without substantial loss of data. This maximizes the overall system efficiency and throughput. High priority and mission critical data may be transmitted to the aircraft using low data rate channels to increase link availability.

FIELD OF THE INVENTION

[0001] The present invention relates to communication systems andmethods employing satellite links, and more particularly to acommunication system employing satellite links using multiplesimultaneous data rates to optimize the data throughput and coverage tomultiple geographically distributed users.

BACKGROUND OF THE INVENTION

[0002] The performance of a communication link between a satellite and amobile platform (i.e., aircraft, ship, train, truck, etc) is influencedby many factors. Most prominently is the effective isotropic radiatedpower (EIRP) of the satellite antenna, in addition to the slant range,rain loss, and Gain over noise temperature (G/T) of the receive antennabeing used to form the link with the satellite. The EIRP of satellitetransponders typically varies across a coverage region, as does theslant range and rain loss. In addition, some antennas, such as a planarphased array antennas, exhibit a large G/T reduction with increasingscan angle. The antenna scan angle, and hence the receive G/T, can varysignificantly with the location and attitude (pitch, roll and yaw) ofthe mobile platform. The result is that the performance of acommunication link from the satellite to the mobile platform can varyover a large range depending on whether the mobile platform is in afavorable or unfavorable location and attitude (relative to thesatellite) in a coverage region. In practice, this performance variationcan be as large as 10× (10 dB) over a coverage region. Performancevariation can become even larger when the mobile receivers use differentsize aperture antennas. Larger antennas provide better link performance.Link performance can be defined in many ways. In this context, it isdefined as the maximum data rate at which the communication link canoperate with a given bit error rate (BER), as described further in thefollowing paragraphs.

[0003] The present discussion refers to mobile platforms that are inless favorable locations and operating with smaller aperture antennas asbeing “disadvantaged”, while mobile users in favorable locations andoperating with larger antenna apertures are referred to as “advantaged”.One factor that determines the performance or degree of favor for aparticular location in satellite coverage region is illustrated withreference to FIG. 1. The EIRP variation for a typical Ku-bandgeostationary satellite transponder (e.g. Telstar 6 at 63° westlongitude) is shown in FIG. 1. Notice that there is about 2 dB variationacross the continental United States (CONUS) coverage area. As mentionedpreviously, other factors can cause a large change in performance acrossa coverage region. Table 1 below shows the effect of slant range andantenna scan angle loss across CONUS. The scan angle loss for a planarphased array antenna manufactured by The Boeing Company is approximatelyequal to cos^(1.2)(θ), where θ is the elevation scan angle to the targetsatellite, measured with respect to an axis extending perpendicular tothe planar aperture. Location Free Space Loss (dB) Antenna Scan Loss(db) Seattle, WA 205.8 3.5 Brownsville, TX 205.3 0.7 Delta (dB) 0.5 2.8

[0004] An analysis can be performed to determine the highest date rateat which a communication link may be operated with a specified bit errorrate (BER). Further to the present discussion, a communication link isconsidered to be “closed” or “available” when it achieves less than somethreshold BER. For this discussion, the threshold BER is assumed to be1E-9, or one erred bit for every billion received. Any excess receivedpower beyond that required to “close” the link is referred to as“margin”. In the present discussion, the term “data rate” will be used,however, an even more accurate term for “data rate” is “informationrate”, which is the available data rate after removing forward errorcorrection (FEC) and other overhead information. Thus, the terms “datarate” and “information rate” will be used interchangeably throughout thefollowing discussion, although “information rate” is, strictly speaking,a more accurate term to describe the available data rate of acommunication link. A user that is in a favorable location within acoverage region is one that can close his communication link at a higherdata rate. Alternatively, a challenged user, or a user in a lessfavorable location within a coverage region, will only be able toachieve communication link closure using lower data rates.

[0005] Referring to FIG. 2, this figure shows contours of the highestdata rates at which links can be “closed” using a Ku-band transponder onTelstar 6 using a Boeing planar phased array receive antenna having anactive aperture measuring 17 inch (43.18 cm)×24 inch (60.96 cm) andhaving 1500 elements mounted flat on the crown of an aircraft flying inlevel attitude. The analysis used to generate this is highlysophisticated and includes the effect of adjacent satelliteinterference. Adjacent satellite interference is caused by the use ofsmall aperture mobile antennas and the elongation of the phased arrayantenna beam that occurs with increasing scan angle. Adjacent satelliteinterference causes further variations in the link performance across acoverage region. The contours are generated by performing a linkanalysis at equally spaced geographic grid points and constructingperformance contours. At each grid point the aircraft is rotated 360° inheading to find the worst case heading. The maximum data rate at whichthe link can be closed for the worst case heading is shown in FIG. 2.Within region A, the maximum channel data rate at which the link can beclosed is 12 Mbps. Within region B a maximum channel data rate of 10Mbps can be used. Within region C a maximum of 8 Mbps, within region D,6 Mbps; within Region E, 4 Mbps; and within region F, a maximum of 2Mbps can be utilized.

[0006] A communication system using a single forward link data ratewould have to operate at a data rate commensurate with the mostdisadvantaged mobile platform in the coverage region. By “forward link”it is meant a signal from a satellite to the mobile platform. Typically,system designers select the highest data rate at which the communicationlink can be closed to the most disadvantaged mobile platform in a givencoverage region. For example, suppose that one wishes to choose a singledata rate for communication across CONUS. FIG. 2 shows that the 6 Mbpscontour covers nearly all of CONUS except for a tiny slice of land innorthern North Dakota. Therefore, 5-6 Mbps would be a good choice forCONUS operation. However, there are regions within CONUS at which thelink can be closed at twice this data rate (i.e., 12 Mbps). Therefore,operation with a single data rate is very inefficient because there aretypically many advantaged mobile platforms that can operate at muchhigher data rates. In other words, the advantaged mobile platforms havelarge excess margins in their forward links which is being wasted when asingle, lower, data rate channel is used to service all mobile platformsoperating within a given coverage region.

[0007] There is also a “coverage vs. capacity” tradeoff associated withthe selection of a single forward link data rate. A low data rate (i.e.,low capacity) permits the link to be closed over a wider coverage area.In contrast, a high data rate is only available in a small coveragearea. FIG. 3 shows the difference in coverage area for 2 Mbps and 8Mbps. The 2 Mbps region is greater than 3 times the area of the 8 Mbpsregion. If multiple data rates could be employed, then both widecoverage and high capacity could be achieved. This is not possible whenoperating with a single data transmission rate.

[0008] Another problem that must be considered is that the mostdisadvantaged mobile platform typically operates with little or nomargin, which means that the communication link is not very robust. Forexample, suppose a disadvantaged mobile platform (e.g., an aircraft)must operate with a forward link data rate chosen so that thecommunication link with the satellite is barely closed. Now suppose thatthe aircraft banks away from the satellite during flight. If theaircraft is using a planar phased array antenna mounted flush on thecrown of the aircraft, then the scan angle to the satellite willincrease and the G/T will decrease. This can cause a loss of thecommunication link. Similarly, the aircraft could stray outside thedesignated coverage region and lose its communication link.

[0009] In summary, problems with the existing “single data rate”approach include capacity inefficiency and lack of robustness (i.e.,lack of margin). The lack of robustness can cause a loss of thecommunication link if the operational environment is adversely affectedsuch as by adverse weather. Rain loss as well as standing water and/orice on the aircraft receive antenna radome or aperture also representssituations where the lack of robustness of a single data rate approachcan compromise the ability to achieve and maintain link closure with amobile platform. The lack of margin also makes it more difficult toinitially acquire the target satellite if the antenna on the mobileplatform is not pointed precisely at the target satellite.

[0010] One method for addressing the above-described problem of managingcommunications links with a number of different mobile platforms capableof operating at varying data rates within a given coverage region couldinvolve the use of a single carrier that is continuously switchedbetween different data rates. Data packets sent to advantaged mobileplatforms could be sent at a higher data rate than to disadvantagedmobile platforms. Making such “on-the-fly” data rate changes requiressignificant time to synchronize the mobile platform RF receiver to eachburst of data that is received at different data rates, resulting in aloss of efficiency. Also, burst mode receivers are far more complex,expensive, and provide reduced performances compared with continuousmode receivers, which are used in the invention.

[0011] Another approach for solving the above-described problem inaddressing multiple mobile platforms capable of communicating atdifferent data rates is the well known “fade mitigation” method. Thismethod is employed with the Advanced Communication Technology Satellite(“ACTS”) operated by the National Aeronautics and Space Administration(NASA). This method involves reducing the information transmission rateduring a rain fade. More specifically, it works by adding forward errorcorrection (FEC) coding during a fade event and removing it during clearweather conditions. Since the bit rate is constant, the addition of FECoverhead reduces the information rate during a fade event and increasesit in clear weather. Such a method could be used to efficiently serviceadvantaged and disadvantaged mobile platforms, except for the fact thatthis approach has insufficient dynamic range. As previously mentioned,the dynamic range between advantaged and disadvantaged mobile terminalsis typically more than 10 dB in a coverage region. FEC will provide atmost only about five dB of dynamic range. Thus, this method would beunsuitable for use in connection with mobile platforms operating withina relatively large coverage region such as CONUS.

SUMMARY OF THE INVENTION

[0012] The present invention is directed to a system and method forproviding a satellite communication link between a base station and aplurality of mobile platforms by using multiple simultaneouscommunication channels operating at different data transmission rates toincrease the link throughput, coverage and reliability. The method andapparatus of the present invention utilizes transmitting data from asatellite to many geographically distributed mobile platforms viamultiple carriers, wherein each carrier forms a single, independentcommunication channel and each of said mobile platforms has the abilityto simultaneously receive multiple channels. The channels are operatedat multiple transmission rates chosen to optimize the throughput,geographic coverage and reliability of the communication links.

[0013] In one preferred embodiment the present invention employs aground based communication system operable to transmit information on aselected one of a plurality of channels, and therefore at a selected oneof a plurality of different information transmission rates. A spacebased transponder system is employed for transponding the informationfrom the ground based communication system over the selected one of theplurality of communication channels to a mobile platform incorporating aplurality of radio frequency (RF) receivers. The selected informationrate/channel is determined in part by the geographic location of themobile platform at any given time within a coverage region. The groundbased communication system selects the maximum information transmissionrate that can be utilized by each mobile platform based in part on themobile platforms position within the coverage region at any given time.This is performed in real time as the mobile platform traverses thecoverage region. The ground based communication system routes datadestined to a particular mobile platform to the communication channeloperating at the highest data rate in which link closure to the mobileplatform is possible.

[0014] In the preferred embodiment each mobile platform incorporates aplurality of receivers, each of which is tuned to a different satellitetransponder channel that is operating at more than one data rate. As themobile platform traverses a coverage region, it will be able to closethe communication link on at least one of the different communicationchannels. Therefore, at least one of the plurality of operationalreceivers on each mobile platform will be successfully receiving data atany time. The ground based system knows which communication channels areviable (which ones can achieve link closure) by real time knowledge ofthe platform's position within the coverage region (as shown in FIG. 2).When the link does not close on a data channel, a high percentage of thereceived data packets contain errors, and the receiver discards thedata. So data must be sent from the ground based system to the mobileplatform using only the communication channels that are closed,otherwise data will be lost. The ground based system decides which one,of the plurality of communication channels being received by the mobileplatform, to send the data destined for that mobile platform. The groundbased system typically selects the highest data rate communicationchannel on which link closure is achieved. In this manner, theinformation transmission rate can be tailored for each particular mobileplatform as it moves between the various subregions of a given coverageregion in a manner which maximizes the overall information transmissionrate so that overall communication efficiency is increased but withoutcausing a loss of data to any particular mobile platform.

[0015] The method and apparatus of the present invention thus allowsmultiple information transmission rates to be used with multiple mobileplatforms without causing a loss of data to any given aircraft, andfurther without under-utilizing the link capacity of any given aircraft.

[0016] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating the preferred embodiment of the invention,are intended for purposes of illustration only and are not intended tolimit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein;

[0018]FIG. 1 is an illustration of a map of the continental UnitedStates (CONUS) showing the EIRP variation for a typical Ku-bandgeostationary satellite transponder (i.e., Telstar 6) at 93° Wlongitude, and showing the variation in dBW across this coverage area;

[0019]FIG. 2 illustrates the various subregions of the continentalUnited States and the maximum information transmission rates (in unitsof Mega bits per second) which can be used in each subregion when theTelstar 6 satellite is used as the space-based component linking groundstation with a mobile platform operating within each of the subregions;

[0020]FIGS. 3a and 3 b are examples of the coverage vs. capacitytradeoff in selecting a forward link information transmission rate, thelower data giving the greater coverage area, and vice versa;

[0021]FIG. 4 is a simplified representation of an exemplarycommunication system which may be used to implement the system andmethod of the present invention;

[0022]FIG. 5 is a detailed block diagram of a mobile terminal used oneach of the aircraft shown in FIG. 4;

[0023]FIG. 6 is a block diagram illustrating multiple communicationschannels used by the present invention for communicating information atdifferent information transmission rates to a pair of mobile terminals;

[0024]FIG. 7 is an illustration of the continental United States (CONUS)illustrating the point of a transmission handoff between the two Mbpscoverage region and the 8 Mbps coverage region during a flight of anaircraft between Seattle and Miami;

[0025]FIG. 8 is a block diagram illustrating the routing of non-missioncritical data packets to the highest data rate channel;

[0026]FIG. 9 illustrates the use of load balancing when considering aselection of channel information transmission rates within a givencoverage region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The following description of the preferred embodiment(s) ismerely exemplary in nature and is in no way intended to limit theinvention, its application, or uses.

[0028] Referring to FIG. 4, there is shown a system 10 in accordancewith a preferred embodiment of the present invention for providing datacontent to and from a plurality of moving platforms 12 a-12 f in one ormore distinct coverage regions 14 a and 14 b. The system 10 generallycomprises a ground segment 16, a plurality of satellites 18 a-18 fforming a space segment 17, and a mobile system 20 disposed on eachmoving platform 12. The moving platforms 12 could comprise aircraft,cruise ships or any other moving vehicle. Thus, the illustration of themoving platforms 12 as aircraft in the figures herein, and the referenceto the mobile platforms as aircraft throughout the following descriptionshould not be construed as limiting the applicability of the system 10to only aircraft.

[0029] The space segment 17 may include any number of satellites 18 ineach coverage region 14 a and 14 b needed to provide coverage for eachregion. Satellites 18 a-18 f are preferably Ku or Ka-band satellites,but could be any frequency from 10 MHz to 100 GHz. Each of thesatellites 18 are further located in a geostationary orbit (GSO) or anon-geostationary orbit (NGSO). Examples of possible NGSO orbits thatcould be used with this invention include low Earth orbit (LEO), mediumEarth orbit (MEO) and highly elliptical orbit (HEO). Each of thesatellites 18 includes at least one radio frequency (RF) transponder,and more preferably a plurality of RF transponders. For examplesatellite 18a is illustrated having four transponders 18 a ₁-18 a ₆(four of which are visible in FIG. 4). It will be appreciated that eachother satellite 18 illustrated could have a greater or lesser pluralityof RF transponders as required to handle the anticipated number ofaircraft 12 operating in the coverage area. The transponders provide“bent-pipe” communications between the aircraft 12 and the groundsegment 16. The frequency bands used for these communication links couldcomprise any radio frequency band from approximately 10 MHz to 100 GHz.The transponders preferably comprise Ku-band transponders in thefrequency band designated by the Federal Communications Commission (FCC)and the International Telecommunications Union (ITU) for fixed satelliteservices FSS or BSS satellites.

[0030] With further reference to FIG. 4, the ground segment 16 includesa ground station 22 in bi-directional communication with a contentcenter 24 and a network operations center (NOC) 26. A second groundstation 22 a located in the second coverage area 14 b may be used ifmore than one distinct coverage area is required for the service. Inthis instance, ground station 22 a would also be in bidirectionalcommunication with the NOC 26 via a terrestrial ground link or any othersuitable means for establishing a communication link with the NOC 26.The ground station 22 a would also be in bidirectional communicationwith a content center 24 a. For the purpose of discussion, the system 10will be described with respect to the operations occurring in coverageregion 14 a. It will also be appreciated that while the system 10 hasbeen illustrated as having two distinct coverage regions, that for thepurpose of the present invention only a single coverage region isrequired. However, it will be understood that identical operationsrelative to the satellites 18 d-18 f occur in coverage region 14 b. Itwill also be understood that the invention may be scaled to any numberof coverage regions 14 in the manner just described.

[0031] The ground station 22 comprises an antenna and associated antennacontrol electronics needed for transmitting data content to thesatellites 18 a and 18 b. The antenna of the ground station 22 may alsobe used to receive data content transponded by the transponders 18 a₁-18 a ₆ originating from each mobile system 20 of each aircraft 12within the coverage region 14 a. The ground station 22 may be locatedanywhere within the coverage region 14 a. Similarly, ground station 22a, if incorporated, can be located anywhere within the second coveragearea 14 b.

[0032] The content center 24 is in communication with a variety ofexternal data content providers and controls the transmission of videoand data information received by it to the ground station 22. Thecontent center 24 may be in contact with an Internet service provider(ISP) 30, a video content source 32 and/or a public switched telephonenetwork (PSTN) 34. The content center 24 may also communicate with oneor more virtual private networks (VPNs) 36. The ISP 30 provides Internetaccess to each of the occupants of each aircraft 12. The video contentsource 32 provides live television programming, for example, Cable NewsNetwork® (CNN) and ESPN®. The NOC 26 performs traditional networkmanagement, user authentication, accounting, customer service andbilling tasks. The content center 24 a associated with the groundstation 22 a in the second coverage region 14 b may also be incommunication with an ISP 38, a video content provider 40, a PSTN 42,and optionally a VPN 44.

[0033] Referring now to FIG. 5, the mobile system 20 disposed on eachaircraft 12 will be described in greater detail. Each mobile system 20includes a data content management system in the form of a router/server50 (hereinafter “server”) which is in communication with acommunications subsystem 52, a control unit and display system 54, and adistribution system in the form of a local area network (LAN) 56.Optionally, the server 50 can also be configured for operation inconnection with a National Air Telephone System (NATS) 58, a crewinformation services system 60 and/or an in-flight entertainment system(IFE) 62.

[0034] The communications subsystem 52 includes a transmitter subsystem64 and a receiver subsystem comprising a plurality of receivers 66.While a plurality of six receivers 66 a-66 f are illustrated, it will beappreciated that a greater or lesser plurality of receivers could beemployed. The transmitter subsystem 64 includes an encoder 68, amodulator 70 and an up-converter 72 for encoding, modulating andup-converting information content signals from the server 50 to atransmit antenna 74. The receiver subsystem 66 includes a decoder 76, ademodulator 78 and a down-converter 80 for decoding, demodulating anddown-converting signals received by the receive antenna 82 into basebandvideo and audio signals, as well as data signals.

[0035] The signals received by the receiver subsystem 66 are then inputto the router/server 50. A system controller 84 is used to control allsubsystems of the mobile system 20. The system controller 84, inparticular, provides signals to an antenna controller 86 which is usedto electronically steer the receive antenna 82 to maintain the receiveantenna pointed at a particular one of the satellites 18, which willhereinafter be referred to as the “target” satellite. The transmitantenna 74 is slaved to the receive antenna 82 such that it also tracksthe target satellite 18. It will be appreciated that some types ofmobile antennas may transmit and receive from the same aperture. In thiscase the transmit antenna 74 and the receive antenna 82 are combinedinto a single antenna.

[0036] With further reference to FIG. 5, the local area network (LAN) 56is used to interface the router/server 50 to a plurality of accessstations 88 associated with each seat location on board the aircraft 12a. Each access station 88 can be used to interface the server 50directly with a user's laptop computer, personal digital assistant (PDA)or other personal computing device of the user. The access stations 88could also each comprise a seat back mounted computer/display. The LAN56 enables bi-directional communication of data between the user'scomputing device and the server 50 such that each user is able torequest a desired channel of television programming, access a desiredwebsite, access his/her email, or perform a wide variety of other tasksindependently of the other users on board the aircraft 12.

[0037] The receive and transmit antennas 82 and 74, respectively, maycomprise any form of steerable antenna. In one preferred form, theseantennas comprise electronically scanned, phased array antennas. Phasedarray antennas are especially well suited for aviation applicationswhere aerodynamic drag is important considerations. One particular formof electronically scanned, phased array antenna suitable for use withthe present invention is disclosed in U.S. Pat. No. 5,886,671, assignedto The Boeing Co., which is hereby incorporated by reference.

[0038] Referring further to FIG. 4, in operation of the system 10, thedata content is preferably formatted into packets before beingtransmitted by either the ground station 22, or from the transmitantenna 74 of each mobile system 20. For the purpose of discussion, atransmission of information (i.e., data) content in the form of packetsfrom the ground station 22 will be referred to as a “forward link”transmission. Packet multiplexing is also preferably employed such thatdata content can be provided to each of the aircraft 12 operating withinthe coverage region 14 a using unicast, transmissions.

[0039] The data content packets received by each of the transponders 18a ₁-18 a ₄ are then transponded by the transponders to each aircraft 12operating within the coverage region 14 a. While multiple satellites 18are illustrated over coverage region 14 a, it will be appreciated thatat the present time, a single satellite (Telstar 6) is capable ofproviding coverage to an area encompassing the entire continental UnitedStates (CONUS). Thus, depending upon the geographic size of the coverageregion and the mobile platform traffic anticipated within the region, itis possible that only a single satellite may be required to providecoverage for the entire region. Other distinct coverage regions besidesCONUS include Europe, South/Central America, East Asia, Middle East,North Atlantic, etc. It is anticipated that in service regions largerthan CONUS, that a plurality of satellites 18 each incorporating one ormore transponders may be required to provide complete coverage of theregion.

[0040] The receive antenna 82 and transmit antenna 74 are eachpreferably disposed on the top of the fuselage of their associatedaircraft 12. The receive antenna 74 of each aircraft receives the entireRF transmission of encoded RF signals representing the data contentpackets from at least one of the transponders 18 a ₁-18 a ₄. The receiveantenna 82 receives horizontally polarized (HP) and vertically polarized(VP) signals which are input to the receivers 66 a-66 f. Each receiver66 a-66 f decodes, demodulates and down-converts the encoded RF signalsto produce video and audio signals, as well as data signals, that areinput to the router/server 50. Data packets that have uncorrected errorsare discarded by the receivers 66 a-66 f and are not passed to therouter/server 50. The router/server 50 filters off and discards any datacontent not intended for users on the aircraft 12 and then forwards theremaining data content via the LAN 56 to the appropriate access stations88. In this manner, each user receives only that portion of theprogramming or other information previously requested by the user.

[0041] Referring to FIG. 6, the ground segment 16 can be seen to includea router 100 and a plurality of RF transmitters 102 a-102 f coupled tooutputs 100 a-100 f of the router 100. The router 100 and the RFtransmitters 102 a-102 f form a router subsystem 103 which preferably isprovided as part of the ground station 22. However, it will beappreciated that it could be provided as a stand alone subsystem or inconnection with other components of the ground system 16, or possiblyeven incorporated in the space based component 17 (i.e., on one of thesatellites 18).

[0042] Preferably, at least one transmitter 102 (102 a) operates at thelowest data rate (in this example 2 Mbps), while one transmitter 102operates at a “medium” data rate (e.g., 6 Mbps), and one transmitter 102f operates at the highest transmission rate (i.e., 12 Mbps). In theexample of FIG. 6, transmitter 102 a may operate at 2 Mbps, transmitter102 b at 4 Mbps, transmitter 102 c at 6 Mbps, transmitter 102 d at 8Mbps, transmitter 102 e at 10 Mbps and transmitter 102 f at 12 Mbps. Itwill be appreciated, however, that a greater or lesser number oftransmitters may be incorporated to accommodate a greater or lesserdynamic range than the 10 db dynamic range provided for by the preferredembodiment of the present invention. In addition, the operational datarates selected for transmitters 102 are dependent on the particularcommunication system parameters.

[0043] Referring further to FIG. 6, for the purpose of explanation,satellite 18 a is provided with six transponders 18 a ₁-18 a ₆. Again,it will be appreciated that a greater or lesser number of transpondersmay be included to meet the needs (i.e., dynamic range, system capacity,etc.) dictated by the geographic size of a given coverage region.Transponder 18 a ₁ is independently associated with transmitter 102 a,transponder 18 a 2 is independently associated with transmitter 102 band so forth. Each of the transmitters 102 a-102 f, in connection withtheir respective transponders 18 a ₁-18 a ₆ thus forms a distinct,independent communication channel over which information is provided ata predetermined information transmission rate.

[0044] Referring further to FIG. 6, two mobile platforms 12 a and 12 bare illustrated in highly simplified form. Each mobile platform 12includes a plurality of receivers 66, and in this example six receivers66 a-66 f, as also explained in connection with FIG. 5. Again, it willbe appreciated that a greater or lesser plurality of receivers can beincorporated on each mobile platform 12. However, the greater the numberof receivers, the greater the number of different communication channelsthat the mobile terminal 20 will be able to receive without the need forre-tuning, and hence the greater the flexibility of the mobile terminal20 in receiving information transmitted at varying informationtransmission rates. The output data streams from receivers 66 a-66 f arecoupled to the input of mobile router 50. The data streams may containdata packets addressed to other aircraft 12 so the router 50 filters offonly those packets addressed to the destination aircraft 12 and discardsthe remainder.

[0045] Each transponder 18 a ₁-18 a ₆ may convey one or more transmitchannels on separate RF carriers, but in the preferred embodiment thereis one spread spectrum channel handled by each transponder. The groundrouter 100 directs data packets to the appropriate output 100 a-100 f sothat the data packets are transmitted over the desired communicationschannel. The ground router 100 may direct data packets to whateverchannel 100 a-100 f is available to a particular aircraft 12 with whichthe ground segment 26 is attempting to transmit information. Again, adisadvantaged aircraft 12 may only have link availability on the lowestinformation transmission rate channel (i.e., in this example the 2 Mbpschannel), while an advantaged aircraft may have link availability on allchannels. The preferred embodiment of the present invention also usesthe ground router 100 to direct all critical data (i.e., all datapertaining to aircraft operation and flight conditions) to the lowestdata rate channel, which in this instance is the 2 Mbps channel. Thiscommunication link is depicted by dash lines 104 in FIG. 6. This lowdata rate link has the highest margin against fading. Amongst the manypossible causes of channel fading are: scan angle loss (e.g., due toaircraft banking, etc.), rain, standing water/ice on the radome, etc.Sending critical data on this link also increases system reliability.

[0046] With further reference to FIG. 6, the preferred embodiment of thepresent invention 10 also routes all non-critical data packets to thehighest data rate channel available to the aircraft 12. This maximizesthe capacity and efficiency of the system 10. A handoff between datachannels must occur for data packets to be redirected through acommunication link operating at a different data rate.

[0047] An example of when the system 10 would hand off communicationsfrom one channel to another is shown in FIG. 7. The Telstar 6 satellite(in this example, satellite 18 a), in conjunction with a Boeing phasedarray receive antenna, produces the data rate contours shown in FIG. 7.Contour 106 corresponds to an 8 Mbps information transmission ratechannel while contour 108 corresponds to a 2 Mbps transmission ratechannel. Within the 2 Mbps contour 108, an aircraft 12 flying in levelattitude above 10,000 feet altitude achieves a greater than 99.9% linkavailability with less than 1E-9 BER while communicating at 2 Mbps.Likewise, within the 8 Mbps contour 106 there is greater than 99.9%availability and less than 1E-9 BER while communicating at 8 Mbps.

[0048] An aircraft 12 flying from Seattle to Miami would start in aregion where only the 2 Mbps channel is available, as defined by contour108, and then would transition into the region defined by contour 106where an 8 Mbps channel is available. Within the area defined by contour106, both the 8 Mbps data channel and the 2 Mbps channel are available.However, to achieve maximum efficiency from the system 10, the highestdata rate channel would be selected by the system 10. The exception forthis would be mission critical data which preferably always uses thelowest data rate channel (i.e., 2 Mbps) for maximum reliability.

[0049] When the aircraft 12 crosses into the 8 Mbps region, the groundrouter 100 switches (i.e., hands off) all non-critical data packets fromthe lower data rate channel (i.e., the 2 Mbps channel) to the higher, 8Mbps, data rate channel. A principal advantage of the present invention10 is that no retuning or reconfiguration is required on the aircraft 12or with the satellite 18 a when a channel handoff occurs. In thepreferred embodiment of the invention, the aircraft receivers 66 a-66 fare tuned to specific transponder frequencies (or channels) when theaircraft 12 enters the service region (i.e. Continental United States)and remain tuned to these channels as the aircraft traverses the serviceregion. When the link is available, data packets will be received on theaircraft 12 by all of the receivers 66 a-66 f. When the links are notavailable, no packets are received in receivers 66 a-66 f. In thepreferred embodiment, the receivers 66 a-66 f discard heavily erredpackets before sending them to mobile router 66. Thus, in the exampleillustrated in FIG. 7, the receiver 66 tuned to the 8 Mbps channel wouldproduce erred data packets when its associated aircraft 12 is outsidecontour 106. The 8 Mbps link is not considered to be available outsideof contour 106 so the ground router 100 would not switch data packets tothe aircraft 12 using the 8 Mbps data transmission rate when it is inthis region. Instead, the ground router 100 would switch data packets tothe 2 Mbps channel when the aircraft is outside of contour 106 andinside of contour 108. Router 50 accepts packets from all receivers 66a-66 f, and filters off those packets destined to that aircraft 12.Thus, the particular one of receivers 66 a-66 f which receives aparticular data packet is irrelevant to the operation of the router 50

[0050] To enable data packets to be handed off from one communicationchannel to another, the ground based router 100 employs a routing tablewhich is updated to accomplish the needed handoff. The updating of thisrouting table can be accomplished in more than one way. A preferredmethod for updating involves using the position and attitude informationreported from each aircraft 12 and using this information to calculatewhether the communication link may be closed on each channel that theaircraft 12 is currently receiving In practice, this operation could betable driven using data rate contour maps such as presented in FIGS. 2and 7. When the aircraft 12 reaches a higher data rate region, therouting table is updated to route non-mission critical packets to thehigher data rate channel and mission critical packets to the lower datarate channel.

[0051] Furthermore, while the contour maps illustrated in FIGS. 2 and 7have been generated while considering level flight of an aircraft, itwill be appreciated that a more sophisticated approach could be employedwhich would consider the attitude of the aircraft in determining whichcommunication channels are available. These additional considerationscould readily be implemented with the present invention 10 because ofthe continuous monitoring of the aircraft position and attitude.

[0052] An alternative method for updating the routing table involvescontinuously checking the availability of the communication channel tothe aircraft 12 using “pings” from a ground based controller 105, asindicated in FIG. 1. The aircraft 12 is required to respond to a “ping”that is received on the forward link channels by replying on its returnlink. This method allows the ground based controller 105 to determinewhich forward link communications channels are available to eachaircraft 12. The drawback to this method is the overhead and complexityassociated with continuously pinging dozens, hundreds or even thousandsof aircraft 12 operating within a given coverage region. While thecontroller 105 is indicated as being associated with the ground station22, it will be appreciated that the controller 105 could be locatedanywhere within the ground segment 16.

[0053] An alternative embodiment of the invention would use one receiver66 that is retuned to different data rate channels as the aircraft 12traverses the coverage region. The aircraft 12 would either have a tablethat defines where to retune or the receiver 66 would periodicallysearch, by cycling through the receive channels, to find the channelwith the highest available data rate. The aircraft 12 would then informthe ground based controller 105 of the channel change. This requires themobile terminal 20 of the aircraft 12 to coordinate with the groundbased controller 105 during a handoff sequence. For this and otherreasons, single receiver operation is anticipated to be less preferablethan using multiple receivers to simultaneously receive informationtransmitted over a plurality of channels.

[0054] Referring to FIG. 8, this is essentially the same configurationof the invention as shown in FIG. 6, except that four transmitters 102a-102 d are used with the ground segment 26, four transponders 18 a ₁-18a ₄ are used with the space segment 17 and four receivers 66 a-66 d areused with the mobile terminal 20 of an aircraft 12. This figureillustrates how the invention, in one preferred embodiment, accomplishesa handoff of non-mission critical data between channels. In thisexample, the aircraft 12 is transitioning to a higher data rate region,so FIG. 8 shows the switching of packets directed to that aircraft 12 inthe ground router from the lower data rate transmitter 102 c to thehigher data rate transmitter 102 d, as indicated by arrow 110. This isaccomplished by changing the routing table of the ground router. Asmentioned previously, the routing tables are automatically updated bythe ground controller based on position reports from the aircraft and ageographic map similar to FIG. 2 that shows the operational regions foreach data rate channel. High priority or mission critical data ispreferably routed through the lowest data rate channel, as depicted bydata path 112 in FIG. 8.

[0055] As previously mentioned, the system 10 is generally useful fortransmitting unicast content. The difference between unicast datatransmission and multicast transmission is that unicast data packets aredirected to individual mobile platforms 12 and multicast transmissionsare directed to multiple aircraft 12 within a coverage region. Sincemulticast data is transmitted to a region, the transmissions must use achannel transmission rate that is available to all aircraft 12 in thatregion. Accordingly, the most disadvantaged aircraft 12 within theregion will determine the maximum data rate at which multicasttransmissions can occur. For example, if the invention were used tomulticast/broadcast within CONUS, then FIG. 2 illustrates that themaximum multicast/unicast data rate is 6 Mbps. If a higher data ratewere to be selected, then not all aircraft 12 would be able to receivethe data. But if a lower data rate were selected, then capacity would bewasted on some aircraft 12. Thus, using multiple data rates provides nobenefit for multicast/broadcast content. However, the invention can beused for transmitting multicast content, but it is likely that thesystem designer would choose to use only a single data rate. In theembodiment illustrated in FIG. 6, any number of the channels could beused to convey multicast/broadcast content.

[0056] Concerning channel transmission rates, the choice of specificchannel transmission rates is preferably based on both achievable linkclosure data rates (as previously described) and the geographicdistribution of users. For example, if it is desired to provide servicein CONUS, where the data rate demand is quite high, and also to provideservice in southern Canada, Mexico and the Caribbean, where theaggregate data rate demand is substantially less, then the system 10 maybe implemented with only two data rate channels. A good choice oftransmission rates based on the contours shown in FIG. 2 is 8 Mbps forthe high demand region within CONUS and 2 Mbps for the low demandregions in Canada, Mexico and the Caribbean. These regions areillustrated in FIG. 9. The 8 Mbps region is indicated by referencenumeral 114 and the 2 Mbps region indicated by reference numeral 116.Within the 8 Mbps region 114 packets can be received at either datarate. However, within the 2 Mbps region 116 but outside of the 8 Mbpsregion 114, defined by reference numeral 118, packets may only bereceived by the mobile platform 12 at 2 Mbps. Therefore, the aggregateload within geographic area 118 should not exceed 2 Mbps for proper loadbalancing.

[0057] A preferred embodiment of the system 10 of the present inventionalso uses closed loop spatial tracking of the target satellite (i.e.,satellite 18 a) based on receive signal strength indications (RSSI) fromthe mobile platform receiver 66. The preferred embodiment uses RSSI fromthe lowest data rate channel for closed loop spatial tracking. Thus, inFIG. 8, the lowest data receiver 66 a would generate a receive signalstrength indication that would be coupled to the tracking system used tokeep the receive and transmit antenna beams pointed at the satellite 18a while the aircraft 12 moves. This provides the maximum margin againstsignal fading and permits disadvantaged aircraft 12 to acquire and trackthe target satellite.

[0058] The system 10 of the present invention thus provides for a meansfor maximizing the efficiency of information transmitted to one or moremobile platforms operating within a given coverage region as the mobileplatform(s) travel throughout the coverage region, and further withoutcausing a loss of the communication link between a ground stationtransmitting information to one or more mobile platforms. Mostadvantageously, no operator intervention is required on the mobileplatforms in order to receive information at different informationtransmission rates. Even further, no configuration (automated or manual)is required on the mobile platform during a handoff betweencommunication channels. Handoffs from one communication channel toanother are accomplished seamlessly as a mobile platform travels withinvarious subregions of a given coverage region. Information transmissionrates are selected which allow the maximum transmission rate to be usedfor information transmitted to any given mobile platform, depending uponthe mobile platform's location and attitude within the coverage region,without being too high to cause a loss of the communication link.

[0059] Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

1. A communication system for transmitting data from a spaced basedsatellite to at least one mobile platform using multiple data ratechannels, comprising; a space based satellite having a plurality oftransmitters operating at a plurality of data rates; said mobileplatforms each having a plurality of receivers, operating simultaneouslyat a plurality of data rates to receive transmitted data from saidtransmitters; said system being operable to transmit data to said mobileplatform on more than one of a plurality of data rate channels andoperating to select a particular said data rate channel for transmittingdata to said mobile platform based on at least a geographic location ofsaid mobile platform; and wherein said system selects a particular oneof said data rate channels for transmitting said data to said mobileplatform in a manner that maximizes overall communication throughput. 2.The system of claim 1 wherein said mobile platform periodicallytransmits position reports concerning its position to said space basedsatellite.
 3. The system of claim 2, wherein said position reports areused to estimate a maximum data rate at which said communication channelcan be operated without substantial loss of transmitted data.
 4. Thesystem of claim 3, wherein said data destined for said particular mobileplatform is switched to a highest one of said data rate channels thatcan be operated without substantial loss of said data being experiencedby said mobile platform.
 5. The system of claim 4, wherein switching ofsaid data is accomplished in a router located in one of said space basedsatellite and a ground based entity.
 6. A space linked communicationssystem comprising: a base communications system operable to transmitinformation on a selected one of a plurality of channels at a selectedone of a plurality of different information transmission rates; a spacebased transponder system for transponding said information received oversaid selected one of said plurality of communications channels; a mobilecommunications system adapted to be carried on-board a mobile platformand being operable to communicate with said transponder system over saidplurality of communications channels; wherein said selected informationrate is determined in part by a geographic location of said mobileplatform within a coverage region; and wherein said selected informationtransmission rate is modified to a different one of said transmissionrates, in real time, as said mobile platform traverses said coverageregion, to thereby increase efficiency and throughput of thecommunications link between said ground based and mobile communicationssystems.
 7. The system of claim 6, wherein said ground basedcommunications system controls the selection of said informationtransmission rate.
 8. The system of claim 6, wherein said mobilecommunications system includes a plurality of receivers, each saidreceiver being tuned to an associated one of said plurality ofcommunications channels.
 9. The system of claim 6, wherein said groundbased segment includes: a plurality of transmitters, each saidtransmitter being adapted to transmit said information over anassociated one of said channels at a pre-selected informationtransmission rate; and a router for directing said information from acontent source to a selected one of said transmitters.
 10. The system ofclaim 9, further comprising a router carried on said mobile platform forreceiving said information received by said receivers to an informationprocessing component carried on said mobile platform.
 11. A space linkedcommunications system for providing a communications link having avariable information transmission rate, said system comprising: a groundbased system having a plurality of transmitters, each of saidtransmitters being adapted to transmit information at a predeterminedinformation transmission rate over a corresponding one of a plurality ofchannels; a satellite based transponder system for transpondinginformation received from said ground based system over a predeterminedgeographic coverage area; a mobile communications system adapted to becarried on a mobile platform and having a receiver system including aplurality of receivers each tuned to an associated one of saidtransmission channels, for receiving said information transponded bysaid satellite based transponder system; wherein an initial informationtransmission rate for said information is determined by said groundbased system based at least on a location of said mobile platform withinsaid geographic coverage area; and wherein said information transmissionrate is varied by said ground based system based at least in part on thelocation of said mobile platform as said mobile platform traverses saidcoverage region to maintain to said information transmission rate at avalue which maximizes the efficiency of said communications link betweensaid mobile communications system and said ground based system withoutcausing said communications link to be broken.
 12. The system of claim6, wherein said ground based system selects said information trans rateto maintain a minimum predetermined bit error rate for said information.13. The system of claim 6, wherein said ground based system includes arouter for routing packets of said information to be transmitted to saidmobile system only to a designated one of said transmitters thereof,thereby providing a unicast transmission of said packets to said mobilesystem.
 14. The system of claim 6, wherein each said informationtransmission rate of said transmitters are associated with apredetermined portion of said geographic coverage area.
 15. A spacelinked communication system for communicating information to a pluralityof mobile platforms travelling within a geographic coverage region, saidsystem comprising: a ground based system for transmitting informationover a plurality of communication channels, each said communicationchannel having a predetermined information transmission rate; a mobilecommunications system disposed on each said mobile platform, each saidmobile communications system being operable to receive information on aplurality of independent channels; wherein said ground based systemoperates to monitor a location of each said mobile platform and selectswhich said communication channel is to be used with each said mobileplatform, based upon the location of each said mobile platform, tomaximize a transmission of said information while maintaining a minimumsignal quality level.
 16. The system of claim 15, wherein said groundbased system comprises a router for routing information to betransmitted to a specific one of said mobile platforms via a specificone of said communication channels.
 17. The system of claim 16, whereinsaid ground based system includes a plurality of transmitters fortransmitting said information over said communication channels.
 18. Thesystem of claim 15, wherein said ground based system controls thetransmission of said information over said communication channels suchthat a predetermined bit error rate is not exceeded for any informationtransmitted to said mobile platforms.
 19. A method for conductingcommunications between a ground based communications system and a mobileplatform, comprising the steps of: having the mobile platform transmitinformation to the ground system indicative of a position of said mobileplatform within a coverage region; using the ground system to determinewhich one of a plurality of information transmission rates to use intransmitting information to said mobile platform, based on the locationof said mobile platform within said coverage region, and selecting aninformation transmission rate being that ensures that a predeterminedtransmission error rate of said information will not be exceeded duringtransmissions of said information; using said ground based system totransmit information to said mobile platform at said selectedinformation transmission rate via a satellite based transponder; andusing a receiver on said mobile platform to receive said information onone of a plurality of communication channels associated with saidselected information transmission rate.
 20. The method of claim 19,wherein said predetermined error transmission rate comprises apredetermined bit error rate for said transmitted information.
 21. Themethod of claim 20, wherein said mobile platform uses one of a pluralityof independent receivers to receive said transmitted information.